In an ad hoc network system, ad hoc network communication terminals (node apparatuses, which are also simply referred to as nodes) autonomously establish network connection to enable mutual communication. The term “autonomously” indicates that there is no need to set a communication route for each user each time, and there is no need for dedicated communication terminals or infrastructure for managing the communication performed by servers and routers.
There are known routing protocols for ad hoc networks (ad hoc protocols) including the reactive-type Adhoc On Demand Distance Vector Algorithm (AODV) and the proactive-type Optimized Link State Routing (OLSR).
In the AODV, broadcasting is used to search for a route, and other communication node apparatuses repeat broadcasting to find a route to the destination node apparatus. A communication node apparatus sends out packets called “Route Request (RREQ)” to its surroundings to find the desired route. The communication node ID of the detection target is specified in this packet.
When a communication node apparatus in the surroundings is not the target of the search, it creates a new RREQ packets and repeats broadcasting to its surroundings. At this time, each communication node apparatus records what the message to the destination has been received from which the adjacent communication node apparatus. When the RREQ message reaches the destination communication node, the destination communication node apparatus creates a “Route Reply (PREP)” packet, and it transmits the PREP to the transmission source node in away in which the PREP follows the route through which the RREQ packet was carried. By doing so, a bidirectional communication route is created.
In the OLSR, communication node apparatuses regularly exchange packets with each other to recognize the overall network, and to detect a route to a destination communication node. The communication node apparatuses periodically send out Hello packets to report their existence to each other. When the communication node apparatuses find the existence of a communication node apparatus to be the counterpart of communication, the communication node apparatuses next create a path for flooding called Multi Point Relay (MPR), to efficiently distribute packets throughout the network. The MPR enables efficient broadcasting of packets from each communication node apparatus to the overall network. Next, the node apparatuses deliver to each other Technology Control (TC) packets which are a route creation message by the MPR, so that all of the node apparatuses are able to recognize the network topology. To transmit a packet to a destination communication node apparatus, the transmission-source communication node apparatus refers to the network topology that it has recognized, to give the packet to the adjacent communication node apparatus to which the packet should be transmitted. The adjacent node apparatus performs a similar process. The packet is eventually delivered to the destination node apparatus.
Node apparatuses belonging to a wireless ad hoc network use Hello packets to propagate routing information.
For example, it is assumed that an ad hoc network includes a node apparatus A and a node apparatus B. The node apparatus A creates and periodically broadcasts to its surroundings a Hello packet that includes routing information stored by the node apparatus A. The node apparatus B that received the Hello packet compares routing information stored in the node apparatus B with the information included in the Hello packet, and the node apparatus B obtains information that has not been stored in the node apparatus B. In addition, the node apparatus B also obtains, from the packet, the quality information about the quality of routes, so as to perform communication using a route with as good quality as possible. Thus, node apparatuses belonging to an ad hoc network use Hello packets to learn information of routes to other node apparatuses included in the network and to estimate the optimal route. Then, communication is performed using the estimated optimal route. Each of the node apparatuses included in the ad hoc network stores the routes to all of the other node apparatuses in the ad hoc network in a routing table. For example, the node apparatus A stores, in its routing table, information of the route to each of the node apparatuses other than the node apparatus A in the ad hoc network. Accordingly, the node apparatus A may transmit a data packet to the node apparatus C via the node apparatus B, for example. These node apparatuses may also be equipped with a weighting table that stores weighting information about the node to be the relay destination for each final destination of packets. In this case, when forwarding a packet, the node apparatuses refer to a weighting table that corresponds to the destination node of the packet to identify a node to be the destination to relay the packet.
However, according to the method in which each node apparatus stores routes to all of the other node apparatuses in an ad hoc network in a routing table, an increase the number of node apparatuses in the network causes an expansion in the size of the routing table. Therefore, there is a known method in which node apparatuses in a network are divided into clusters, and each of the node apparatuses stores information of routes to node apparatuses in the same cluster in its routing table.
There is a method in which a control packet is used when generating a cluster from node apparatuses in a network. For example, according to route information, each node determines a cluster to join, and each node transmits and receives a control packet when participating in a cluster and when leaving a cluster. Generally, another cluster is formed when forming a cluster and the number of nodes in the cluster exceeds a prescribed value.
In addition, there is also a known method in which a Hello packet is used instead of the control packet.
In an ad hoc network system, there is a problem in that when failures such as hardware faults, radio wave troubles, power outages and the like occur during the operation of the system, there is no means to recover from functional failures in the cluster. For example, there is a problem in that it is impossible to maintain the cluster formation function and the cluster merger function in the normal way. There is a problem in that continued functional failures in the cluster lead to an increase in the number of clusters in the overall network which causes an increase in the number of what a route extend over different clusters when routing. Accordingly, many processing resources are required, which leads to a lower throughput.
[Patent document 1] Japanese Laid-open Patent Publication No. 2011-193341